A depolymerized natural rubber obtained by depolymerization of natural rubber is also called liquid rubber. Having strong adhesive power and excellent crosslinking reactivity, it has been used widely as a raw material for adhesives, pressure-sensitive adhesives, sealing materials, caulking compounds and the like. In addition, it has also been used as a reactive plasticizer for improving processing properties of solid rubber such as compounded rubber for tires.
It is now attracting attention in various industrial fields, because as compared with solid rubber, liquid rubber is advantageous for the production of various products from the viewpoint that it can be easily processed and requires less energy.
In general, a depolymerized natural rubber can be obtained by mastication, pyrolysis, photolysis, chemical decomposition or the like of the natural rubber but the depolymerized natural rubber obtained by such methods has problems, which will be described below, respectively.
Mastication is a method for accelerating reduction in the molecular weight by breaking the rubber molecular chains of the raw material rubber through mechanical action and heat in a roller mill or internal mixer, and then adding a peptizing agent (which is an organic compound giving plasticity even in a small amount of addition, thereby reducing mastication time and is also called a mastication accelerator) such as a mercaptan to the resulting rubber to prevent the recombination of the broken molecular chains. This method permits the production of depolymerized rubber having a low molecular weight, but is accompanied with the problem that the molecular weight distribution is wide and therefore cannot be controlled easily. In pyrolysis, the reaction temperature is high so that not only decomposition but also crosslinking or recombination inevitably occurs, which makes it difficult to control the molecular weight distribution.
Photolysis is a method for breaking the molecular chains with light energy such as ultraviolet light. This method is accompanied with the drawbacks that peroxide or the like at the terminal of ketone, which has been formed by the break of the main chain, causes decomposition successively, serving as a sensitizer so that it is difficult to control the molecular weight or molecular weight distribution and moreover, isomerization reaction tends to proceed from a cis-1,4-structure to a trans-1,4-structure.
As chemical decomposition, ozone degradation, oxidative degradation by an oxidizing agent such as hydrogen peroxide or phenylhydrazine, and oxidative degradation by a metal ionic catalyst are known. Ozone degradation is applied only as an analysis means and industrially, is of no value, because the reaction should be conducted at a low temperature and treatment of a large amount of ozonide is dangerous.
Depolymerized natural rubber making use of oxidative degradation using hydrogen peroxide or phenylhydrazine has already been industrialized. The molecular-weight-reduced natural rubber latex introduced by "Polymer Digest, October issue, 90(1981)" is not the one in which liquefaction has proceeded sufficiently and it takes a solid form. A similar example can also be found in JP-A-58-152075 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and the depolymerized natural rubber disclosed therein, which rubber has been obtained by adding hydrogen peroxide to a natural rubber latex and then reacting them while blowing oxygen into them, is a solid rubber having a breaking strength of 3.5 kg.
In "Rev. Gen. Caoutch Plast. Vol. 61, No. 643, 79(1984)", a technique is introduced for obtaining a depolymerized natural rubber having a molecular weight of 3,000 to 20,000 by depolymerizing a natural rubber latex using phenylhydrazine and air. As is apparent from "Makromol. Chem. Vol. 186, No. 12, 2441(1985)", its terminal group is phenylhydrazine which does not have the reactivity necessary for the extension of the main chain. It is also reported in "Makromol. Chem. Rapid Commun. 7, No. 3, 159(1986)" that liquid natural rubber which has a molecular weight of not greater than 10,000, has a Mw/Mn ratio of 1.6, 1.7 or the like, that is, a narrow molecular weight distribution, and has a hydroxyl group at its terminal can be obtained by adding hydrogen peroxide and methanol or tetrahydrofuran to a solution of natural rubber in toluene and then exposing the resulting mixture to ultraviolet rays. It is however reported in "Makromol. Chem. Vol. 189, No. 3, 523(1988)" that as a result of the supplementary test after that, it was found that the intramolecular epoxide group was mistaken for the terminal hydroxyl group.
According to the experiments made by the present inventors, epoxidation and isomerization of the main chain occur upon depolymerization by exposure to ultraviolet rays under the similar conditions and moreover, the formation of functional groups such as carbonyl group, carboxyl group or hydroxyl group was not recognized at the terminal of the molecular chains. Furthermore, the depolymerized natural rubber so obtained had so large molecular weight distribution, that is, Mw/Mn (a weight-average molecular weight/number-average molecular weight ratio, this will apply equally hereinafter) of at least 4 so that its control was difficult.
For producing a crosslinked substance having good rubber elasticity, it is preferred that the molecular weight distribution be as narrow as possible.
The conventional depolymerized natural rubber is accompanied with the problem that because natural rubber, which has not been subjected to deproteinization and therefore has a low purification degree, is used as a raw material, the conventional depolymerized natural rubber cannot be free from the odor peculiar to natural rubber and in addition, is colored owing to the oxidation or the like of protein. Therefore it is difficult to use it for the applications frequently exposed to the public eye, for example, pressure-sensitive adhesives for our familiar products such as daily necessities or nursing products or for albums which are requested to be colorless and transparent. Furthermore, it cannot be used for the products which come in contact with the human body, because there is a potential danger of it causing immediate allergy derived from protein, which has been one of the recent problems.